In power generation and other industries, it often becomes necessary to attach nozzles (such as replacement heater nozzles) or other devices to pressure vessels. Generally, these nozzles are welded in place with a strength weld on the outside of the vessel and with a seal weld on the inside of the vessel. However, applying a strength weld (one requiring the addition of filler material) on both sides of the vessel is also a common practice, it just being a more complicated procedure.
As can be imagined, the weld on the outside of the vessel is quite accessible. Unfortunately, the same cannot be said of the inside weld since access to the inside of the vessel is often limited to the small opening of the nozzle itself. This is due to the potential radiation fields or other hazards or limitations on access commonly encountered within such vessels.
To make these inner welds, an internal seal welder was developed which provides a method for remotely making the inside weld from the outside of the vessel. This seal welder reaches up through the nozzle with a gas tungsten arc welding torch, normally referred to as a TIG torch, which is manipulated to follow the curved path around the nozzle end. The exact path to be followed (circular, ellipsoid, etc.) is a function of both the circular nozzle end and the angle of its intersection with the inside of the pressure vessel. It was thus hoped that the seal welder would consistently and repetitively yield an acceptable weld.
Unfortunately, however, the use of a TIG welding torch introduces many more variables which must be understood and controlled before such consistency can be achieved. These variables include such items, among others, as a properly sharpened tungsten tip, consistent weld joint geometry, and consistent torch to workpiece distance. Additionally, in the event an interior strength weld is required (as compared with a seal weld), filler metal will also need to be supplied to the weld which presents its own set of problems in the confined space within the nozzle. Furthermore, the weld occurs some radial distance from the longitudinal axis of the nozzle, thus the welding torch and the filler metal will need to be angled and supplied accordingly.
It is thus an object of this invention to provide a means for seal welding as well as strength welding a replacement nozzle to the interior of a vessel from its outside. Another object of this invention is to utilize plasma arc welding techniques thereby avoiding the problems normally associated with the TIG process. A further object of this invention is to provide powdered metal filler rather than weld wire when strength welding is required. Still another object of this invention is to provide a means of welding which can better accommodate different joint geometry configurations and variations so that the torch-to-workpiece distance becomes less critical. Another object of this invention is to reduce set-up time, weld contamination and noise that are associated with TIG welding. A further object of this invention is to incorporate a smaller heat-affected zone than is normally possible with TIG welding thereby affecting (i.e. reducing the hardness of) a smaller portion of the surrounding material. These and other objects will become obvious upon further investigation.